A relatively mild autumn and early winter have put less stress on the region’s aging water pipes, causing the total number of water main breaks for much of the area to drop in 2011.

Barring a severe temperature swing that would tax pipes in the next two weeks, Montgomery and Prince George’s counties are on pace to have the lowest number of water main breaks since 2006 and one of the lowest annual tallies of the past decade, according to figures from the Washington Suburban Sanitary Commission (WSSC).

As of Dec. 19, Montgomery and Prince George’s have had 1,440 leaks or breaks, down about 650 from last year, according to WSSC figures. Without a sudden influx of breaks, the WSSC would finish out 2011 with the fewest number since the 1,251 repaired in 2006. The past four years have hovered around 2,000 breaks.

DC Water has logged 309 breaks, compared with 440 last year. Fairfax Water has had nearly 300, about 90 fewer than last year.

But local water utilities’ officials aren’t celebrating yet. The improved figures, they said, reflect the luck of the thermometer more than the overall health of their underground pipe systems, parts of which date back a century.

The annual number of water main breaks “bounces all over the place” from one year to the next, said Gary J. Gumm, chief engineer for WSSC. “But the overall trend is up.”

This year’s lower tally “just means that our pipes haven’t been put under the same stress [this year], even though they’re a year older.”

Gumm said WSSC computer models show it will take 12 more years of stepped-up pipe replacements to reverse the trend in the Maryland suburbs. WSSC has increased pipe replacements from 16 miles in fiscal 2007 to 44 miles last fiscal year. The utility plans to replace 55 miles annually by fiscal 2015.

Friday marks the third anniversary of a massive WSSC water main break that required motorists to be rescued from a torrent of frigid water on River Road in Bethesda. The break highlighted a national problem of aging underground pipes failing after years of neglect. The River Road pipe hadn’t been inspected in a decade before it burst.

Hundreds of smaller pipe breaks across the Washington region every year draw less attention, but they waste water, damage local roads, snarl commutes and affect water service.

Water utility officials said it’s not the cold that stresses underground mains as much as major temperature swings that cause a pipe to repeatedly expand and contract.

Last December saw a surge in broken mains when 23 days of that month fell below the average high temperature, including a stretch of several days when the high temperature fluctuated by 12 degrees. This month has been far milder with few topsy-turvy changes.

In the District, the number of breaks has hovered around 400 for the past three years.

Olu Adebo, the utility’s chief financial officer, said he has seen a downward trend since the 538 breaks in 2007. While the weather remains a primary predictor of breaks, “even in the years with extreme weather and spikes, we’re not spiking as high,” Adebo said.

The District’s water pipes average 75 years old, he said.

DC Water has been replacing about four miles of pipe — one-third of 1 percent of its 1,300 miles — every year for the past decade, Adebo said. The utility is ramping up to replace 13 miles (or 1 percent of the system) annually by fiscal 2015.

At that pace, it will take a century to replace the entire system, DC Water officials said, but it will be triple the previous rate.

“I expect to see even better results after that,” Adebo said.

Fairfax Water officials said that their pipes benefit from their relatively young age: Most are 50 years old or less.

“That makes a big difference when you’re talking about pipe breaks,” Fairfax Water spokeswoman Jeanne Bailey said.

Officials at the utilities said they’ll keep their eyes on the thermometer. This year’s biggest WSSC break occurred three weeks into the new year, when a 54-inch pipe blew apart near the Capital Beltway, destroying a church and two businesses at a Capitol Heights office park and leaving much of the area under a boil-water advisory for several days.

There hasn’t been anything like that this season.

But “we’ll see how the rest of the winter goes,” Gumm said.